Class vs Interface in TypeScript: Some Key Differences

Overview

In TypeScript, classes and interfaces are powerful structures that facilitate not just object-oriented programming but also type-checking and design contracts.

Comparing classes and interfaces in TypeScript reveals both syntactical and conceptual differences vital for developers to understand. This article explores these distinctions and provides insights into when each should be used. TypeScript, as a superset of JavaScript, enhances the language by introducing types and providing extensive support for classes and interfaces—concepts originating from object-oriented programming paradigms.

Definition of Class and Interface

A class in TypeScript is a blueprint for creating objects. A class encapsulates data for the object and methods to manipulate that data. On the other hand, an interface is a way to define a contract in your application; it is a custom type that defines properties and methods signatures without implementing the details.

Declaration and Instantiation

Let’s start with how classes and interfaces are declared and how instances are created from them:

Aspect	Class	Interface
Declaration	A class is declared with the class keyword.	An interface is declared with the interface keyword.
Instantiation	Classes are instantiated with the new keyword.	Interfaces cannot be instantiated on their own; they function only as a typing contract.

Implementation Details

While a class contains both a definition and an implementation, an interface is purely a declaration. Here’s a closer look:

	Class	Interface
Properties	Can have full properties and readonly properties.	Can only have readonly properties or property signatures.
Methods	Includes actual method implementations.	Only method signatures are defined—no implementations.
Constructors	Can have constructors to initialize instances.	Interfaces do not have constructors as they are not instantiable.

Inheritance vs. Extension

One of the main structural differences between classes and interfaces is how they extend functionality. For classes, it’s about inheritance, while interfaces are about extension.

Aspect	Class	Interface
Inheritance	Uses extends keyword. Can extend another class, inheriting all its members.	—
Extension	—	Uses extends keyword. Can extend other interfaces or even classes, but does not implement them—only inherits the types.

Access Modifiers

Classes in TypeScript support access modifiers such as public, private, and protected, whereas interfaces do not. They augment encapsulation in classes, impacting how properties and methods can be accessed:

Access Modifier	Class	Interface
Public	The default access level. Members are free to be accessed from anywhere.	All interface members are inherently public; no modifiers are used.
Private	Members can only be accessed within the class that defines them.	Not applicable to interfaces.
Protected	Members can be accessed within the class and by the inheriting class instances.	Not applicable to interfaces.

Practical Application and Best Practices

In practical terms, deciding between using a class or an interface often depends on whether implementation details are needed and the purpose of the type’s usage within the code. Interfaces are typically used for defining custom types without tying them to any particular implementation. Classes are full-fledged actors in the code, participating in instantiation and runtime behavior.

It’s crucial for TypeScript developers to discern when to use a class versus an interface. An interface promotes loose coupling and high cohesion in software design, which is excellent for defining adaptable APIs and contracts. On the other hand, a class encompasses an entire entity with behaviors and states suitable for creating comprehensive instances to be manipulated and interacted with in your application logic.

Conclusion

In summary, TypeScript’s use of classes and interfaces serve distinct yet complementary roles. Understanding their differences is essential to architecting robust, type-safe applications. Harnessing each construct’s features will enable developers to harness the full power of TypeScript’s capabilities in software development.